A semiconductor diode laser is a monocrystalline pn junction device. In one form of such a device, the pn junction is in a plane disposed in an active region between two parallel rectangular faces of a monocrystalline semiconductor body. Two mutually parallel reflective faces that are perpendicular to the pn junction form a laser cavity. Lasing action is produced by applying a forward voltage across the pn junction. The forward bias injects electrons and holes across the pn junction. Electrons and holes recombine in the active region to cause stimulated emission of the radiation. Above a given level of electron injection, called the threshold current (I.sub.TH), emitted radiation is collected and amplified in the active region. The amplified radiation exits the active region parallel the pn junction as a monochromatic beam.
A problem is that electrons and holes can be injected into the active region without stimulating emission therein. For example, they can escape outside the active region to adjacent portions of the semiconductor body, where they recombine without contributing to laser emission. Analogously, photons produced in the active region can escape from the active region by radiation in a direction not parallel the pn junction. In addition, it is possible for electrons to disappear within the active region without producing the desired emission of radiation, such as by combining with holes at crystal defects. All such losses reduce laser efficiency, i.e., output power. One can resist escape of injected electrons and holes and stimulated photons from the active region by sandwiching the active region between two contiguous layers of monocrystalline semiconductive material having a larger energy band gap and a lower index of refraction than the active region. Such layers serve to confine electrons, holes and photons to the active region. On the other hand, the active region, and as a practical matter the two contiguous layers, must be of a very high monocrystalline quality. This requires that these layers and the active region be closely matched not only in crystal structure but also in crystal lattice size. Moreover, one of the sandwiching layers must be doped to n-type conductivity and the other to p-type conductivity. Such a structure is referred to herein as a double heterojunction semiconductor diode laser.
Lead chalcogenide double heterojunction semiconductor diode lasers which operate at high temperatures have been difficult to make. By high temperature I mean higher than about 100.degree. K. under continuous wave (CW) operation. I have previously filed U.S. patent application Ser. No. 565,397 on a quaternary semiconductive diode laser system based on an alloy of lead-europium-selenide-telluride which permits such lasers to be made with relative ease.
I have also previously filed a U.S. patent application Ser. No. 754,171 entitled "Lead-Alloy-Telluride Heterojunction Semiconductor Laser" which describes two quaternary compositions, lead-europium-calcium-telluride and lead-strontium-calcium-telluride, either of which compositions may be used to make diode lasers which operate at high temperatures. However, both of these alloys contain calcium (Ca), and the energy band gap of these alloys (and hence the laser emission wavelengths of diode lasers made from these alloys) depend very sensitively on the calcium concentration of these alloys. It is more difficult to measure the amount of Ca very accurately that is put into these alloys because Ca is a relatively light atomic species. The technique I used for measuring alloy composition during growth by molecular beam epitaxy involves weighing vacuum deposited thin films of each film constituent separately as I have described in a publication (see D. L. Partin, J. Electronic Materials, vol. 13, pp 493-504, 1984). Thus, if the relative weights of the appropriate molecules or atoms used in the various alloys I have grown are considered, they are (rounded off to nearest whole number): PbTe (335), PbSe (286), GeTe (200 ), Te.sub.2 (255), Eu (152), Yb (173), Ba (137), Sr (88) and Ca (40). Thus, Ca is more than twice as light as the next heavier specie, and in practice, I found this to cause a fair amount of difficulty, especially because the energy band gap of Pb.sub.1-x Ca.sub.x Te is a sensitive function of Ca concentration. Thus, I find an advantage in controlling the energy band gap of Pb.sub.1-x Sr.sub.x Se.sub.y Te.sub.1-y compared to other alloys containing Ca.